DISCUSSION: Dunion et al. (2014) used IR satellite data to show that tropical cyclones tend to have a blowup of convection (cold cloud tops) near their center around sunset that subsequently propagates outward over the course of the following day. IR satellite data is sensitive primarily to cloud top. We wanted to see if this diurnal cycle was also present through a deeper layer of the storm's clouds below cloud top. Hence, in Leppert and Cecil (2016), we used active and passive microwave satellite data that can observe below cloud top to find that, indeed, this tropical cyclone diurnal cycle in cloud cover/rainfall extends through a deep layer. In the figure above, the top-left shows a snapshot of IR satellite data over Hurricane Hilary valid 0300 LST 24 July 2017. You can see cold cloud tops over the center of the storm and areas of cold cloud tops almost in a ring a little farther out from the center. The top-right shows the difference in IR brightness temperatures between the time of the image in the top-left and 6 hours prior showing the change in brightness temperature with time. The yellow-orange-red ring shows cooling brightness temperatures with time or the outward propagating diurnal pulse which corresponds with the ring of cold brightness temperatures in the top-left image. The bottom of the image show the expected radial location of this diurnal pulse as a function of time.
What is remarkable is that this diurnal pulse shows up consistently in storm after storm and in tropical cyclones around the world. Similar figures to that above can be found here for current storms. This feature seems really interesting, but we are not really sure its importance at this time. There is some indication that the blowup of convection is related to a similar increase in intensity. This diurnal cycle may also be related to changes in storm size. But, more research is needed to understand the importance of this tropical cyclone diurnal cycle.
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ⓒ 2017 Meteorologist Dr. Ken Leppert II